The present invention relates to a method for making formulated fertilizers, optionally with microelements, in a spherical granule form having a high granulometric homogeneity, start from urea-formaldehyde resins in liquid form, and to the fertilizers made thereby.
Actually, also the fertilizers made by the inventive method constitute an integrating part of the invention.
As is known, urea-formaldehyde (UF) resins in solid form and of low molecular weight, are fertilizers adapted to slowly release nitrogen, both due to a high contents of said nitrogen (>36%), and to their capability of gradually continuously releasing nitrogen for a direct absorption by plants and the like.
This latter feature is a very important both from an economical and environmental standpoint.
Available fertilizers are compounds having a very high water solubility, and accordingly these compounds can be easily flushed or washed away by rain and irrigation waters.
In turn, this flushed away material can contaminate waters and ground water tables and moreover, since the flushed away material cannot aid the plant growth, it must be supplemented by new fertilizer compounds.
The efficiency of UF resins based nitrogenous fertilizers, including UF resins in solid form, is measured by the so-called “Activity Index” (AI) parameter, which is related to the respective amounts of hot water (HWIN) and cold water (CWIN) insoluble resins, and is bound to the nitrogen releasing rate as follows:AI=100·(1−HWIN/CWIN).
Small values of the above mentioned parameter AI are indicative of a slower releasing, whereas high values are indicative of a quicker releasing.
A standard nitrogenous fertilizer would have an AI value from 40% to 60%.
Prior urea-formaldehyde condensation products, containing low water solubility methylene-urea, have a nitrogen amount contents which can be released to plants within a long time period.
In turn, UF resins, used as a nitrogenous fertilizer, have a comparatively high value, both due to their high nitrogen contents, and their nitrogen releasing characteristics, both in cold and hot water.
The water soluble longer chains, on the other hand, can be hydrolized much more slowly than shorter chains.
The releasing efficiency abruptly decreases for oligomers including a urea amount larger than 5 units (tetramethylenepentaurea) and, at such an amount, they become nearly unattachable by the soil microorganisms.
Many commercially available fertilizers based on solid UF resins have a high hot water insolubility nitrogen rate, (HWIN), thereby this nitrogen, accordingly, would be practically unavailable for plants.
Thus, for reducing the hot water insoluble nitrogen amount, while holding the cold water insoluble fraction, thereof nearly unaltered, two methods have been conventionally used.
The first of these methods provides to add to the urea/formaldehyde mixture a further urea amount.
By this method, however, the excess free urea causes an increase of the fertilizer phytotoxicity.
The second method, in turn, provides to add ammonia in the form of ammonium salts in order to favour a formation of low molecular weight condensation products, in general not higher than trimers (dimethylenetriurea).
The condensation reaction of urea and formaldehyde is usually carried out by two stages:                1) in the first stage, urea and formaldehyde, in suitable ratios from 1.3 to 1.2 and at a pH from 7 to 10, are mixed at environment or room temperature, thereby favoring formation of methylolurea.        2) in the second stage, the mixture is acidified and heated (at a pH from 4.5 to 6.0 and at a temperature from 80 to 110° C.), thereby providing a UF resin.        
The reaction can also be carried out in a single stage, by acidifying at low temperature (20÷40° C.) for a long time; however, such a method would provide a product very rich in free urea, with consequent phytotoxicity problems as the resin is used as a fertilizer.
An alternative method could comprise a use of an alcohol as a reaction solvent (either methyl or ethyl alcohol).
The urea-formaldehyde condensation reaction is always carried out by acidifying methylolurea formed in a basic environment or medium, the temperature being, in such a case, controlled by boiling the solvent off, since said solvent would operate as a thermal “flywheel”.
More specifically, the condensing/drying reaction can be directly carried out in a granulating device, for example of a fluid bed type, or in a spraying-drying tower or column or, alternatively, in a CFSR (Continuous Flow Stirred Reactor) reactor, to exit therefrom as a suspension to be supplied to a turbo-dryer or other like apparatus.
Alternatively, the liquid form can be adsorbed on inert inorganic materials (vermiculite and clays in general).
For making fertilizers in grain form starting from a foamed resin, the condensing reaction is carried out by acidifying in the presence of surface active materials (0.05÷2%) and/or air in a reactor including a suitable stirring device.